(Prior Art)
In an operation environment surrounding a diesel engine, a regulation is becoming severe in recent years, and it is desired to provide a diesel engine having a performance which can satisfy the regulation. Japanese Unexamined Patent Publication No. 63-134844 describes an exhaust gas recirculation type internal combustion engine. In this publication, in order to avoid an increase of an NOx discharging amount caused by a reduction of an EGR gas amount, there is disclosed a technique of controlling so as to secure a predetermined amount or more of EGR gas. The invention described in No. 63-134844 relates to the internal combustion engine in which a throttle valve is necessarily provided as a constituting element, as described in claim 1 of No. 63-134844. In other words, although No. 63-134844 describes the internal combustion engine, the embodiment is described by exemplifying a gasoline engine, and the throttle valve is described in FIG. 1. Accordingly, it is apparent that the diesel engine is not included in the internal combustion engine expressed in the invention of No. 63-134844.
No smoke (no soot) is discharged from the gasoline engine disclosed in the publication mentioned above, and it is basically sufficient to take a structure for preventing NOx from being discharged into consideration. On the contrary, in the diesel engine, if an EGR gas amount is increased, an NOx amount is reduced, but a discharge amount of the smoke is increased. Accordingly, in the diesel engine, it is impossible to uphold a control standard for the exhaust gas even by applying the invention described in the patent document 1.
In the exhaust gas recirculation type diesel engine, at a time of controlling the EGR gas amount, it is necessary to regulate within a range which can simultaneously suppress both the NOx amount and the smoke amount discharged.